Compressed air foam systems (CAFS) have been used in firefighting for quite some time. The original CAFS developments were made by the U.S. Navy during the 1940's as a method to fight fires on board ships. In its most basic form, CAFS is simply a means for mixing air and water with a surfactant in order to produce a water-based foam that is used to extinguish fires. CAFS provides quicker “knockdown” against potent fires, deeper penetration of fuels, and gives firefighters the advantage of making their initial attack against a fire from a significantly greater distance than with a traditional water stream or fog pattern. The bubble structure allows for greater expansion of delivered water surface area, allowing for greater heat reduction compared to equal amounts of plain water. Foam blankets allow for pre-treatment of fuels that are not already involved in the fire, and have less adverse impact on property, as well as helping to prevent damage to evidence used for fire investigations. In fact, some studies have indicated that CAFS increases the effectiveness of water as an extinguishing agent by approximately a factor of five. CAFS may be particularly valuable for rural fire departments, because the use of foam reduces the amount of water required to extinguish a fire, and rural departments are often quite limited in the amount of water that they have available at any particular fire.
A persistent challenge with CAFS design is the maintenance of a correct balance between incoming water and air pressure. If these pressures are not correctly balanced, a low-quality foam or no foam at all may result. Most firefighting vehicles equipped with CAFS use a centrifugal water pump to provide “boost” water pressure control, and a rotary screw air compressor to provide air pressure. Both of these devices are driven by the vehicle's engine through a transmission gear box. It will be seen from this arrangement then that the vehicle's engine rotations per minute (RPMs) will control the pressure of the incoming water and air flows. A balancing valve is added with feedback controls in order to maintain the appropriate water and air pressure ratio. By automatically adjusting engine RPMs in response to the balancing valve feedback, a high-quality foam output may be achieved. A number of manufactures provide such systems as described above, including Hale Products Inc. of Conshohocken, Pa.
The existing commercial CAFS are designed to be used in conjunction with a low-pressure water source. It is generally accepted in the industry that the water source for CAFS should be at about 20 pounds per square inch (psi) or less for satisfactory performance at the water pump intake. Thus the typical source of water for a CAFS is a water tank on board the CAFS vehicle or other on-site firefighting vehicle. Such systems cannot be effectively employed with direct feed from a high-pressure source, such as a typical fire hydrant. The reason for this is that the centrifugal pump used to create water pressure in such systems boosts the incoming water pressure, and the result is that the vehicle's engine RPM necessary for satisfactory water pressure will be insufficient to produce the required volume of air for production of satisfactory CAFS. Thus if the incoming water pressure from the water source is higher than approximately 20 psi, the system would be incapable of producing a properly balanced, high-quality firefighting foam because there is no means of lowering the incoming water pressure with a centrifugal pump.
One means to connect existing commercial CAFS to a high pressure source is to provide a relief valve to bleed off incoming water pressure. This is suggested, for example, in the User Operation Manual for Hale's “CAFSPro” system. But the result of using a bleed-off relief valve is wasted water, which may be a particularly critical concern for rural fire departments, where utilization of all available water may be essential for many firefighting efforts. Otherwise, as the CAFSPro system User Operation Manual explains, “[i]f incoming water pressure is excessively high, it may not be possible to maintain engine RPM and desired pressure.” Another means is to attempt to gate the incoming water stream to match the required volume, but this is not an acceptable means, for if a nozzle at any discharge is closed or opened it adversely affects the system pressure, so the pump will either be starved for water and have insufficient volume and pressure, or a pressure spike will occur that is above an acceptable threshold before the pump operator can adjust the incoming flow.
To address these limitations of existing commercial CAFS, firefighters today typically feed water from a hydrant source into a CAFS by connecting the hydrant to a vehicle water tank, and then connecting the vehicle water tank to the CAFS. But this approach requires that a firefighter manually open and close valves continuously during operation in order to maintain a proper level in the vehicle water tank, or that an automatic system be installed that requires a series of valves with level sensors and actuators. Commercial automated systems for this purpose include Hale's “AutoFill” tank fill valve. The manual approach to water tank level management ties up a firefighter that could otherwise perform other critical tasks, which is an especially important issue for rural fire departments with limited manpower. The automated approach requires the addition of expensive equipment, which again is a particular concern for rural fire departments since the price of such equipment may not be within their limited equipment budgets. In addition, automated equipment of this type includes many moving parts, which requires regular maintenance in order to reduce the likelihood of a critical failure in the field. What is desired then is a simple, inexpensive, reliable means for limiting the incoming water pressure from a high-pressure source, such as a hydrant, in order that it may be reliably used with CAFS to produce high-quality foam.